106 CHEMISTRY OF NATURAL WATERS. [IX. 



of soda or sulphate of magnesia enter largely. The soda-salt 

 may sometimes be formed by the reaction between solution of 

 gypsum and natriferous silicates referred to in 7, or by the 

 decomposition of gypsum by solution of carbonate of soda; 

 while in other cases its origin will probably be found in the 

 natural deposits of sulphates, such as glauberite, thenardite, 

 and glauber-salt, which occur in saliferous rocks. A similar 

 origin is probable for many of those springs in which sulphate 

 of magnesia predominates. This salt also effloresces abundant- 

 ly in a nearly pure form upon certain limestones, and is in 

 some cases due to the action of sulphates from decomposing 

 pyrites upon magnesian carbonate or silicate. In by far the 

 greater number of cases, however, its appearance is unconnected 

 with any such process ; and is, according to Mitscherlich, due 

 to a reaction between dolomite and dissolved gypsum. 



20. In support of this view, it was found by the chemist 

 just named that when a solution of sulphate of lime was made 

 to filter for some time through pulverized magnesian limestone, 

 it was decomposed with the formation of carbonate of lime and 

 sulphate of magnesia. This reaction I have been unable to 

 verify. A solution of gypsum in distilled water was made to 

 percolate slowly through a column of several inches of finely 

 powdered dolomite, and after ten nitrations, occupying as many 

 days, no perceptible amount of sulphate of magnesia had been 

 formed. Solutions of gypsum were then digested for many 

 months with pulverized dolomite, and also with crystalline 

 carbonate of magnesia, but with similar negative results ; nor 

 did the substitution of a solution of chloride of calcium lead 

 to the formation of any soluble magnesian salt. Solutions of 

 gypsum were then impregnated with carbonic acid, and allowed 

 to remain in contact with pulverized dolomite and with magne- 

 site, as before, during six months of the warm season, when 

 only inappreciable traces of magnesia were taken into solution. 

 These experiments show that no decomposition of dissolved 

 gypsum is effected by native carbonate of magnesia, or by the 

 double carbonate of lime and magnesia, at ordinary tem- 

 perature. 



